1. Field of the Invention
The present invention relates to an image forming apparatus that transfers unfixed images to recording materials by a transfer means, and fixes the unfixed images on the recording materials by a fixing means.
2. Description of the Related Art
Conventionally, as shown in FIG. 14, an electrophotographic type image forming apparatus transfers unfixed images borne by an intermediate transfer belt 102 to recording materials P by a transfer roller pair 103, fixes the unfixed images on the recording materials P by a fixing roller pair 105, and discharges the recording materials on which the images have been fixed to a discharge tray 107 outside the apparatus, by a discharge roller pair 111.
The fixing roller pair 105 heat-fixes the unfixed images to the recording materials. However, the heat temporarily removed from the fixing rollers through the fixing process varies depending on amount of deposit of developer that forms unfixed images transferred to the recording materials, the number of passing recording materials per unit time that passes through the fixing roller pair, difference in heat capacity of the recording materials. A temporary temperature change that takes place in the fixing roller pair 105 appears as a change of size of the outer diameter of the rollers. This might lead to a difference between the conveyance speed of the recording materials by the fixing roller pair 105 and the conveyance speed of the recording material by the transfer roller pair 103.
Such a difference between the conveyance speeds of the recording materials of the fixing roller pair 105 and the transfer roller pair 103, if any, might result in a deterioration of the image. In other words, when the conveyance speed of the recording materials by the fixing roller pair 105 is faster than that of the recording materials by the transfer roller pair 103, the recording materials are stretched between the fixing roller pair 105 and the transfer roller pair 103, causing image disturbance or the like, thereby leading to image deterioration when images are transferred to the recording materials P by the transfer roller pair. On the other hand, when the conveyance speed of the recording materials by the fixing roller pair 105 is slower than that of the recording materials by the transfer roller pair 103, the recording materials form an oversize loop between the fixing roller pair 105 and the transfer roller pair 103. Then, the recording materials P that form the oversize loop are pressed hard and scraped against a conveyance guide 120, and the scraping against the conveyance guide 120 might cause image disturbance, thereby resulting in deteriorated images when they are transferred. Thus, the following proposal has been made previously for solving the problem of image deterioration as described above, by forming a predetermined loop on recording materials P between the transfer roller pair 103 and the fixing roller pair 105 and thus preventing the recording materials P from being stretched or bending too much.
As shown in FIG. 14, a loop detecting sensor 12 which detects a loop of recording materials P is provided on the conveyance guide 120 between the fixing roller pair 105 and the transfer roller pair 103. Based on result of the detection, the conveyance speed of the recording materials by the fixing roller pair 105 is switched to a first speed that is slower than the conveyance speed of the recording materials by the transfer roller pair, or to a second speed that is faster the first speed, so that the predetermined loop can be maintained. Such the technology is disclosed in Japanese Patent Application Laid Open (JP-A) No. 05-107966 Publication.
In order to maintain recording materials in a predetermined loop, however, the fixing roller pair is switched to the first speed or the second speed so that a predetermined loop can be maintained. Thus, there is a fear that noise would result from the switching of the conveyance speed of the fixing roller pair. In addition, there is also another fear that poor images might occur, such as image blurring at the transfer portion or fixing nonuniformity at the fixing device, due to periodic minor vibration caused by the switching operation of the conveyance speed.